饿汉式：
public class A {
private static A a = new A();
private A() {
}
public static A getInstance() {
return a;
}
}
懒汉式：（线程不安全）
public class A {
private static A a = null;
private A() {
}
public static A getInstance() {
If a == null {
A = new A();
}
return a;
}
}
懒汉式：（线程安全方式但不推荐）
public class A {
private static A a = null;
private A() {
}
public static synchronized A getInstance() {
If a == null {
A = new A();
}
return a;
}
}
懒汉式：（线程安全方式但有几率出错，因为对象的内存申请和初始化不同步的原因）
public class A {
private static A a = null;
private A() {
}
public static synchronized A getInstance() {
synchronized (A.class){
If a == null {
A = new A();
}
}
return a;
}
}
静态内部类方式：（推荐）
public class A {
private static class AHolder {
private static A a = new A();
}
private A (){}
public static A getInstance() {
return AHolder.a;
}
}
